1. Field of the Invention
The present invention relates to an image processing device and system, and more particularly to an image processing device and system which picks up an image from an original document and processes the image.
2. Description of the Related Art
Typically, in a scanning operation, available scanners project a light ray onto an original to be scanned, the light ray is reflected from the original and then enters optical block of the scanner, and the level of the light ray is analog-to-digital converted (hereinafter A/D converted) into a digital value that is output as scanned image information. One of the important parameters that determines the image quality of such a scanner is contrast of the image. The contrast of the image is defined as the range between the darkest pixel in the image and the lightest pixel in the image. The scanner A/D converts acquired analog image data into digital data within the range of contrast. The contrast information is represented relative to a white level reference, namely, digital data that is obtained when the optical system of the scanner scans a white reference surface. Specifically, when analog image data from each pixel is A/D converted, white reference data is used as a reference.
When the scanner reads the image and the white level reference, its output is determined depending on the hardware such as a light source, an optical sensor, an A/D converter circuit. Also determined are position accuracy and variations among individual scanners. When reading any given pixel, the rate of variation dependent on these factors should be kept to a minimum to obtain a reliable output from the pixel and then achieve excellent image quality. To this end, a diversity of methods are implemented: for example, high precision components are used, the light intensity of the light source is increased, the read time per pixel is increased, and measurement of white level as a reference is performed at each scanning of the original.
When the scanner scans the original through a clear sheet, light transmissivity of the clear sheet lowers reflectance of the original, and thus, a contrast correction circuit corrects the reflectance or the white level reference is updated each time a scan is completed.
When the miniature size and low cost of the scanner are dictated by portability, space requirement, and economy considerations, components in use are subject to limitations: for example, components having high precision, heat resistance, and high noise immunity, or electric circuits containing such components, are difficult to employ. The output level of the light source cannot be increased from the standpoint of power saving, low-cost design, and heat dissipation. For this reason, light sources with a small light intensity, such as a low-cost Xe lamp and an LED, are frequently used. To acquire data in a reliable fashion, the read time per pixel (accumulation time per pixel) is prolonged, and the gain of the sensor is increased. These all contribute to the increase in the cost of the scanner. Occasionally, miniaturization of the scanner results in a scanner which cannot physically accommodate a white level measuring surface (white surface) for measuring the white level reference. In such a case, a user must place a white level standard sheet on an original document mount to measure the white level reference and scan it.
Some wordprocessors today are equipped with a serial printer in which a print head cartridge, as recording means, can be replaced with a scan head cartridge that is similarly sized to the print head cartridge. In this way, wordprocessors with such a scanner unit mounted can function as a scanner device as well.
Unlike a normal flat head scanner, a serial scanner has a scanner-unit mounted carriage that is designed to travel a relatively long distance, and thus scan time is prolonged. Because of the long scan time, an increase of accumulation time of a sensor such as a CCD is difficult to implement. Furthermore, acquisition of the white level reference takes time accordingly.
When scanning is performed through a clear sheet, physical characteristics of the clear sheet degrade the reading accuracy and gain of the sensor. Because of this, the serial scanner must be periodically calibrated in connection with the white level reference measurement using the white level standard sheet. The serial scanner takes time to complete scanning, making frequent calibrations impractical and resulting in a degradation in image quality.